bisect: avoid copying ancestor list for non-merge commits
During a bisection, hg needs to compute a list of all ancestors for every
candidate commit. This is accomplished via a bottom-up traversal of the set of
candidates, during which each revision's ancestor list is populated using the
ancestor list of its parent(s). Previously, this involved copying the entire
list, which could be very long in if the bisection range was large.
To help improve this, we can observe that each candidate commit is visited
exactly once, at which point its ancestor list is copied into its children's
lists and then dropped. In the case of non-merge commits, a commit's ancestor
list consists exactly of its parent's list plus itself. This means that we can
trivially reuse the parent's existing list for one of its non-merge children,
which avoids copying entirely if that commit is the parent's only child. This
makes bisections over linear ranges of commits much faster.
During some informal testing in the large publicly-available `mozilla-central`
repository, this noticeably sped up bisections over large ranges of history:
Setup:
$ cd mozilla-central
$ hg bisect --reset
$ hg bisect --good 0
$ hg log -r tip -T '{rev}\n'
628417
Test:
$ time hg bisect --bad tip --noupdate
Before:
real 3m35.927s
user 3m35.553s
sys 0m0.319s
After:
real 1m41.142s
user 1m40.810s
sys 0m0.285s
#require no-windows
$ . "$TESTDIR/remotefilelog-library.sh"
$ hg init master
$ cd master
$ cat >> .hg/hgrc <<EOF
> [remotefilelog]
> server=True
> EOF
$ echo x > x
$ echo z > z
$ hg commit -qAm x1
$ echo x2 > x
$ echo z2 > z
$ hg commit -qAm x2
$ hg bookmark foo
$ cd ..
# prefetch a revision w/ a sparse checkout
$ hgcloneshallow ssh://user@dummy/master shallow --noupdate
streaming all changes
2 files to transfer, 527 bytes of data (no-zstd !)
transferred 527 bytes in * seconds (* */sec) (glob) (no-zstd !)
2 files to transfer, 534 bytes of data (zstd !)
transferred 534 bytes in * seconds (* */sec) (glob) (zstd !)
searching for changes
no changes found
$ cd shallow
$ printf "[extensions]\nsparse=\n" >> .hg/hgrc
$ hg debugsparse -I x
$ hg prefetch -r 0
1 files fetched over 1 fetches - (1 misses, 0.00% hit ratio) over *s (glob)
$ hg cat -r 0 x
x
$ hg debugsparse -I z
$ hg prefetch -r 0
1 files fetched over 1 fetches - (1 misses, 0.00% hit ratio) over *s (glob)
$ hg cat -r 0 z
z
# prefetch sparse only on pull when configured
$ printf "[remotefilelog]\npullprefetch=bookmark()\n" >> .hg/hgrc
$ hg strip tip
saved backup bundle to $TESTTMP/shallow/.hg/strip-backup/876b1317060d-b2e91d8d-backup.hg (glob)
2 files fetched over 2 fetches - (2 misses, 0.00% hit ratio) over *s (glob)
$ hg debugsparse --delete z
$ clearcache
$ hg pull
pulling from ssh://user@dummy/master
searching for changes
adding changesets
adding manifests
adding file changes
updating bookmark foo
added 1 changesets with 0 changes to 0 files
new changesets 876b1317060d
(run 'hg update' to get a working copy)
prefetching file contents
1 files fetched over 1 fetches - (1 misses, 0.00% hit ratio) over *s (glob)
# Dont consider filtered files when doing copy tracing
## Push an unrelated commit
$ cd ../
$ hgcloneshallow ssh://user@dummy/master shallow2
streaming all changes
2 files to transfer, 527 bytes of data (no-zstd !)
transferred 527 bytes in * seconds (*) (glob) (no-zstd !)
2 files to transfer, 534 bytes of data (zstd !)
transferred 534 bytes in * seconds (* */sec) (glob) (zstd !)
searching for changes
no changes found
updating to branch default
2 files updated, 0 files merged, 0 files removed, 0 files unresolved
1 files fetched over 1 fetches - (1 misses, 0.00% hit ratio) over *s (glob)
$ cd shallow2
$ printf "[extensions]\nsparse=\n" >> .hg/hgrc
$ hg up -q 0
2 files fetched over 1 fetches - (2 misses, 0.00% hit ratio) over *s (glob)
$ touch a
$ hg ci -Aqm a
$ hg push -q -f
## Pull the unrelated commit and rebase onto it - verify unrelated file was not
pulled
$ cd ../shallow
$ hg up -q 1
$ hg pull -q
$ hg debugsparse -I z
$ clearcache
$ hg prefetch -r '. + .^' -I x -I z
4 files fetched over 1 fetches - (4 misses, 0.00% hit ratio) over * (glob)
$ hg rebase -d 2 --keep
rebasing 1:876b1317060d foo "x2"